1. Field of the Invention
The invention relates to aircraft flight path control apparatus particularly with respect to flight path transition control.
2. Description of the Prior Art
One type of prior art flight path transition control apparatus utilizes the control law Y+KY=0. This is the control law conventionally utilized to control an aircraft along a wings-level straight line path where Y is the lateral displacement of the aircraft from the path, Y is the rate of change of Y and K is a time constant of the system. When the aircraft deviates from the straight line course to which it is being controlled, the Y and KY terms become non zero and the difference therebetween generates a roll command .phi..sub.c to return the aircraft to the straight line path. When, in such prior art systems, it is desired to execute a transition from a first straight line course to a second straight line course, the lateral displacement Y and the displacement rate Y with respect to the first course is switched to the displacement and displacement rate with respect to the second course. An error signal proportional to the difference between Y and KY generates a roll command .phi..sub.c to roll the aircraft into a turn in order to capture the second straight line course.
In such prior art systems it is often desirable to execute an exponential transition path from the first course to the second course. It is appreciated that the above mentioned control law Y+KY=0 is the differential equation defining the desired exponential path. When the aircraft is on the desired exponential transition path the difference between the lateral displacement Y of the aircraft from the new course and the displacement rate KY will be 0. Since the control law Y+KY=0 is utilized to command the aircraft roll attitude to turn the aircraft toward the new path, the expression Y+KY is not equal to zero during the transition but to the non-zero roll attitude command .phi..sub.c required to maintain the aircraft in a banked attitude so as to continue turning toward the new path. Since Y+KY=0 defines the desired exponential transition path, but in fact, Y+KY=.phi..sub.c, the aircraft does not follow the desired exponential path but instead flies a standoff path with respect to the desired exponential curve with a resultant undesired overshoot of the new straight line course. In such prior art systems it was necessary to hold an error signal between Y and KY in order to maintain the aircraft in a roll attitude resulting in the offset from the desired exponential path with the concomitant overshoot of the new course. Utilizing the control law .phi..sub.c =Y+KY to control the roll attitude of the aircraft so as to turn toward the new path guarantees an offset from the desired exponential path and an overshoot of the new path. It is appreciated that the roll command signal .phi..sub.c may be applied to the roll channel of either an automatic flight control system or of a flight director system to provide either automatic or directed human pilot flight control in executing the transition.
An alternative prior art transition control apparatus may be found in U.S. Pat. No. 3,994,456, issued Nov. 30, 1976 to William C. Post and Edmond E. Olive entitled "Steered Lateral Course Transition Control Apparatus for Aircraft Area Navigation Systems" and assigned to the assignee of the present invention. In this area navigation (RNAV) system the aircraft is controlled to follow a circular transition path from a first straight line course to a second straight line course. The steered turn control apparatus of said U.S. Pat. No. 3,994,456 suffers from an undesirable roll removal transient condition when approaching the new path. At a predetermined distance from the new path the bank angle command that determines the circular transition path is reduced to zero in order to roll the aircraft out onto the new path. The removal of the bank angle command, however, causes the aircraft to deviate from the desired circular path so that the actual path is not tangent to the new course. This condition results in an undesirable overshoot of the new course.
It is appreciated that with respect to the prior art exponential path capture and the RNAV path capture discussed above, lag in establishing the bank angle so as to execute the transition path again results in a deviation from the desired transition path.